Drawing inspiration from the sociocultural turn in language teacher cognition research, this conceptual article argues for the utilisation of cultural-historical activity theory (CHAT) as a theoretical framework for researching and understanding teacher cognition as a social phenomenon. In this article, three CHAT-related concepts, namely "mediation," including "contradictions"; "emotion"; and "agency" are expounded, drawn together, and proven as pertinent and useful analytical tools for the study of the complexity of teacher cognition. Its central thesis posits that CHAT accommodates teachers' sociocultural activity systems whose mechanism is characterised by mediation that stimulates their emotional responses and agency for taking actions--a process through which teacher cognition unfolds in dynamic, developmental manners. By mapping these concepts, the article contributes to the extant scholarship by highlighting language teacher cognition as a mediated, emotional, and agentive process of transformation and providing critical epistemological implications for prospective empirical attempts in this line of research.